The present invention relates to blasting operations and particularly to blasting operations conducted in conjunction with mineral mining, for example, coal mining in either surface or underground mines. In surface mining operations it is usually necessary to remove overburden to uncover the mineral deposits, for example, the coal seam. When the overburden is composed of loose soil or unconsolidated rock-like materials it can easily be removed by conventional earth moving equipment. In contrast, when the overburden consists of highly consolidated soils or rock, for example limestone or sandstone, blasting operations must ordinarily be employed to fragment the material before it can be removed. Sometimes such blasting operations can also be caused to throw or cast the overburden material into the previously excavated pit. This obviously saves the time and expense of equipment removal of the fragmented overburden from the coal seam.
Blasting operations in surface coal mines can induce high level ground vibrations and airborne noise. When surface mining operations are located near critical structures, e.g., homes, buildings, towers, pipelines, etc., structural damage from blast induced vibrations and noise is a possibility. In addition, the subjective evaluation of noise and vibration by the community as being either structurally damaging or an environmental nuisance can lead to complaints, bad relations, and litigation. Because of this, blasters must either follow strict (and quite conservative) laws limiting charge weight, or they must monitor each shot to prove their compliance with vibration regulations. These constraints affect the manner in which the blast is conducted and require extra time, equipment, personnel, and explosives, all of which impact the industry economically.
Blasting operations input large amounts of impulsive energy into the surrounding earth medium. A portion of this energy is transmitted away from the blast region in the form of vibrational waves. If the excitation frequencies of a multiple-charge blast coincide with preferential ground response frequencies, then even small excitation amplitudes can excite amplified ground motions. If ground response frequencies coincide with resonance frequenices of nearby structures, as is frequently the case in coal overburden blasting operations, amplified structural motions and possibly structural damage can occur. For houses, whole structure resonances occur roughly between 4-10 Hz, and wall resonances occur roughly between 10-25 Hz. Exact frequencies depend upon structure geometry and material properties.
Notwithstanding the considerable work and studies already done in this field, there still remains a substantial need for improved methods and apparatus which can better control the amplitude, frequency, and duration of blast induced vibrations and noise. Further, this must be of a sort which can be easily transferred to operating companies so that field personnel may use it both for preventing structural damage, and for meeting community concerns as well. In addition, a need remains for improving the effectiveness of the overburden removal process, such as by increasing the fragmentation, swell, and cast of the overburden material thrown from the coal seam.
It appears that most studies reported in the blasting literature, while providing some understanding of the basic excitation and vibration phenomena, are of limited value for making accurate amplitude or frequency predictions. In addition, the measurement and analysis methods seem limited in light of the signal processing techniques which are in common use in other fields of engineering and geophysics.
Two notable exceptions were the studies of Siskind, et al, (Siskind, D. E., Stagg, M. S., Kopp, J. W., and Dowding, C. H., 1980, "Structure Response and Damage Produced by Ground Vibration from Surface Mine Blasting," Bur. of Mines Report of Investigations, 8507) and Anderson, et al, 1982, (Anderson, D. A., Winzer, S. R., and Ritter, A. P., 1982, "Blast Design for Optimizing Fragmentation while Controlling Frequency of Ground Vibration," Proc. 8th Conf. Explosives and Blasting Technique, Soc. Explosives Engrs.). The evolution of understanding in this field is well covered in the bibliographies of these two publications, as well as that of Anderson, et al, 1985, (Anderson, D. A., Ritter, A. P., Winzer, S. R., and Reil, J. W., 1985, "A Method for Site-Specific Prediction and Control of Ground Vibration from Blasting," Proc. 11th Conf. Explosives and Blasting Technique, Soc. Explosives Engrs.).
The Bureau of Mines (BOM) conducted an exhaustive empirical study which attempted to account for the frequency, charge weight, and distance effects of blast vibrations on structural response and damage (Siskind, et al,). However, this study represents many different overburden and construction material types, and the data is spread over 1-2 orders of magnitude. Hence, these results cannot be used to accurately predict the maximum vibrational velocity of the overburden material, referred to as the peak particle velocity (PPV) amplitude, for a particular mine site since the effects of excitation, ground, and structural frequencies on the data presented are not known. The best that can be done is to use the BOM data to make extremely conservative estimates of PPV amplitude which will guarantee that regulatory limits are not exceeded.
Hence, the Office of Surface Mining (OSM) (Office of Surface Mining, 1983 "Surface Coal Mining and Reclamation Operations; Permanent Regulatory Program; Use of Explosives; General Performance Standards; Permit Application," Fed. Reg., Mar. 8, 1983 adopted blasting vibration regulations based upon the BOM study which allow three methods for a blasting operation to demonstrate compliance, i.e., the maximum overall PPV method, the scaled-distance method, and the PPV-frequency chart method. If a company can show by measurement and analysis that they possess an accurate PPV prediction method which allows them to use larger charge weights than allowed by OSM regulations, then they may blast using the higher charge weights per delay. A primary objective of the present invention is to provide this improvement.
Only recently have investigators in this field realized that proper firing delay control might be used to control vibration frequencies, minimize vibration amplitudes and durations, assist in reinforcement or cancellation of stress waves, and optimize material fragmentation (Anderson, et al, 1982). Although the necessary electrical equipment for controlling the frequency (delay periods) of blast excitation impulses exists, little use has been made of this approach to date.
Recently, Anderson, et al, (1985) reported a technique for predicting site specific ground vibrations excited by blasting operations. See also Anderson, et al, 1983 (Anderson, D. A., Winzer, S. R., and Ritter, A. P., 1983, "Synthetic Delay Versus Frequency Plots for Predicting Ground Vibration From Blasting," (Proc. 3rd Inter. Symp. on Computer-aided Seismic Analysis and Discrimination, IEEE). This technique involves measuring a ground response time history to a single-charge blast and then repeatedly superposing (adding) this time-domain signal, delayed in time, to construct a synthetic multiple-charge response time history. This procedure is repeated for a large range of delay period intervals. Ranges of amplitude levels of the predicted multiple-charge signals are then mapped on a delay versus frequency plot. The best delay period, consistent with currently accepted blasting practice, is then selected for suitably reducing vibrations at critical ground frequencies. This superposition approach implies that the ground response is linear, i.e., has relatively small deflections, an assumption which it is believed does not hold in general, especially for large charge weights per delay such as those used in explosive fragmentation and casting operations in surface coal mines.
The nonlinear effect was briefly mentioned by Anderson, et al, (1985) and deferred to a later presentation. However, Winzer, et al (Winzer, S. R., Anderson, D. A., Ritter, A. P., Holloway, D. C., and Thomas, H. E., 1983, "A Study of the Mechanisms of Fragmentation and Vibration in Blasting Operations," Martin Marietta Laboratories Final Report, U.S. Dept. of Energy Contr. No. DE-ACO1-80ET 14357) noticed a strong nonlinear effect in quarry blasts for some charge weight, delay period, and hole spacing combinations. This paper further states (p. 23) that this rendered the linear superposition approach unacceptable. Obviously, therefore, there are potentially important inconsistencies in the prior art.